The present invention relates to the heat-sealing of polymer films in general, and more particularly relates to a heating element for a film-sealing apparatus, and to a method for making such a heating element.
Conventional devices for making longitudinal heat seals in polymer film material generally include a sealing wire or other electrical heating means wrapped around and affixed to the circumference of a rotatable cylinder such that the wire rotates along with the cylinder. To effect a heat seal, the entire wire is heated, by sending an electrical current therethrough, and the cylinder is brought into rotational contact with the film plies to be sealed as the film plies move against and past the heated cylinder. While this approach has generally been effective, its main drawback is that consistent sealing temperatures are difficult to achieve.
Such inconsistent sealing temperatures are due to two primary factors: first, the entire wire is heated and, second, the wire is wrapped about the entire circumference of the rotatable cylinder. During continuous operation of the apparatus, this configuration results in a gradual but steady increase in the temperature of the cylinder because the cylinder continually absorbs heat from the wire at a faster rate than the rate at which the heat can be dissipated to the air. Without careful control and monitoring of the longitudinal heat-seal device, the ever-changing temperature of the rotating cylinder often results in inconsistent seals being made between the film plies. As a consequence, insufficient heat necessary to form complete (strong) heat-seals may be applied at the beginning of a session, and/or an excessive amount of heat may be applied toward the end of the session, which causes the cylinder to melt through the film plies and produce holes in the film. When the heat-seal device melts through the film plies, an outer strip from one or both film plies very often separates from the rest of the film and wraps around the longitudinal heat-seal device. This results in the necessity of shutting down the apparatus and extricating the film strip from the heat-seal device. Typically, the strip is tightly wound around the device and/or partially melted such that removal of the strip is a difficult and time-consuming process.
Another drawback of the “wrap-around” type of heat-seal device as discussed above is that routine maintenance procedures are more cumbersome and time-consuming than would otherwise be desired. In particular, the sealing wire is difficult to remove and replace when worn.
Finally, since the entire wire must be heated, the energy requirements for the sealing device are higher than desired.
The above problems have been addressed in U.S. Pat. No. 6,550,229, assigned to the assignee of the present application and incorporated herein by reference in its entirety. The '229 patent discloses a device for sealing two plies of plastic film together, comprising an electrically conductive heating element having a first end secured to a first node and a second end secured to a second node, and a rotatable support cylinder having an outer circumferential surface, the cylinder being positioned in abutting relationship with the heating element and capable of rotating against the heating element along a predetermined contact arc such that less than a complete circumference of the outer surface of the cylinder is in contact with the heating element. Accordingly, two juxtaposed plies of plastic film may be sealed together by bringing the juxtaposed film plies into contact with the heating element along the contact arc and causing sufficient current to flow through the heating element that the heating element heats to a sealing temperature that causes the juxtaposed film plies to seal together.
The heating element in one embodiment disclosed in the '229 patent has a relatively short heating wire segment of nickel-chromium alloy having a relatively high electrical resistance. One end of the heating wire segment is butt-welded to a stainless steel conductor lead, and the opposite end of the heating wire segment is butt-welded to another stainless steel conductor lead. The conductor leads have lower electrical resistance than the heating wire segment, such that the heating wire segment heats to a higher temperature than the conductor leads for a given amount of current flowing through the heating element. The heating wire segment is in contact with the rotatable support cylinder along the contact arc, such that the juxtaposed film plies are contacted by the heating wire segment. By suitably regulating the current, the heating wire segment heats to the sealing temperature while the conductor leads remain below the sealing temperature. In this manner, undesired melting of the film plies by the conductor leads is avoided.
The sealing device disclosed in the '229 patent represented a significant advance over the then-existing state of the art. Nevertheless, further improvement is sought. In particular, it is desired to increase the linear advance rate of the juxtaposed film plies being sealed together.